1. Field of the Invention
The present invention relates to dryers, and more particularly to a fluidized-bed dryer.
2. Prior Art
Fluidized-bed dryers are grouped into two types: continuous and batch. In continuous fluidized-bed dryers, operation is continued without interruption as long as wet feed or material is successively supplied. A common problem with this continuous type of dryer is that the particles of the succeeding wet feed are mixed with the particles of the preceding wet feed suddenly in the drying chamber, causing non-uniform drying due to the difference between the periods of stay of the particles within the drying chamber.
Batch fluidized-bed dryers operate in the following sequence: (1) wet feed is supplied to the drying chamber; (2) the wet feed is dried, during which time no additional wet feed is supplied; and (3) dry product is discharged from the drying chamber. In this type of dryers, all the particles of wet feed within the drying chamber are dried for a constant period of time, thus causing uniform drying. However, batch fluidized-bed dryers have the following problems, because peripheral equipments or apparatus also must be batch.
1. The succeeding wet feed must be temporarily stocked outside the drying chamber, ready to be supplied, until drying of the preceding wet feed is finished and, subsequently, dry product is discharged from the drying chamber. If the wet feed is adhesive material, the particles of the wet feed in stock would collect into lumps which would tend to be objectionably deformed or otherwise broken due to their own weight during handling. Because of this lumping and misshaping of the particles, adhesive wet feed could not be easily fluidized during drying, and scattering of dust-like particles (of the wet feed) would occur in the drying chamber, resulting in unstable operation and hence inadequate quality product.
2. When a large amount of wet feed (generally, wet feed is poor in fluidity) is supplied at once to the drying chamber, the particles would collect into lumps due to poor fluidity and would stick to a porous supporting shelf and wall surfaces in the drying chamber, causing unstable operation.
3. When wet feed containing alcoholic, acetone or similar organic solution is dried, such solution would vapour suddenly at the beginning of drying and, as a result, there would be a danger that the gas concentration in the drying chamber increases beyond its critical value to explode. For safety, it is necessary to retard the rate of vaporization by lowering the temperature of heated air at the beginning of drying; in this condition, efficient drying cannot be achieved.